Various types of elements for providing signals through a line in various configurations are known. For example, U.S. Pat. No. 6,204,524 ('524), incorporated herein by reference, describes CMOS active pixel sensor (APS) arrays and compares them to other semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices, and hybrid focal plane arrays.
In many known configurations, a number of light sensing elements, often called “pixels”, provide their signals through a conductive line, sometimes referred to herein as a “readout line”. In a row/column array, for example, each column typically has a readout line to receive signals from a group of pixels that includes one pixel from each row; when a row is selected, each pixel in the row can provide its signal through its column's readout line. As used herein, in a row/column array, a “row” is defined as a line of pixels or other signal-providing elements that can be concurrently selected for readout, while a “column” is defined as a line of pixels that provide their signals through a shared readout line, also referred to as a “column line”.
FIG. 1 shows circuit 100, which exemplifies features of a conventional CMOS APS array as exemplified in '524. Circuit 100 can be implemented as integrated circuitry on a chip. Pixel array 110 is a row/column array with M rows and N columns of pixels. As illustrated, pixel 112 is in one of the rows and one of the columns; while its row is selected, pixel 112 can provide signals through line 114, the readout line for its column.
Within pixel 112, photo-generated charge converted from photons of radiant energy is transferred to region 120, and can be amplified during transfer. Region 120, also referred to as a floating diffusion region, is in turn connected to provide a signal to the gate of source follower transistor 122 indicating a quantity of charge in region 120. Source follower transistor 122 converts the gate signal to a pixel output voltage. When Row switch 124, which can also be a transistor, is closed by a row select signal, the pixel output voltage results in a signal to readout circuitry 130 through line 114.
Within readout circuitry 130, sample and hold (S/H) circuit 132 includes load transistor 134 biased by voltage VLN to provide a bias current of appropriate magnitude for source follower transistor 122 through line 114. During sampling, S/H transistor 136 can be turned on and, in response to the pixel output voltage from source follower transistor 122, S/H capacitance 138 stores a voltage representing the amount of charge in diffusion region 120. The voltage stored by S/H capacitance 138 can then be used to provide a signal to readout path 140 for further processing. In a typical implementation, S/H circuit 132 for line 114 includes two separately switched S/H capacitances, one for signal sampling and one for reset sampling, as shown in '524.
In circuit 100 and other circuits, problems can be caused by undesired currents, such as photocurrents, that affect signals provided through lines. The term “spurious current” is used herein to mean any current on a line that, although not a genuine bias current, affects a signal provided by a pixel or other signal-providing element through the line.
Although spurious current could result from various undesired processes, it is especially problematic in a CMOS image sensor implemented in silicon. In a CMOS image sensor, some photoelectrons generated by silicon are not captured by the photodiode or other photosensitive region of any pixel. Instead, such photoelectrons can produce an undesired photocurrent on certain other nodes in the pixel circuitry. In particular, a very bright object can cause a significant photocurrent at the row select transistors of many pixels in a column.
In circuit 100 in FIG. 1, for example, bright light can cause photocurrent to flow from line 114 through a parasitic diode at the open Row switch of pixel 112. As a result, the performance of source follower transistor 122 may be affected.
The main effect of photocurrent from bright light involves bias current provided by load transistor 134. The bias current is modified by photocurrent in the Row select switches of pixel 112 and of other pixels in the same column; the photocurrents may, in combination, be comparable to the bias current. Therefore, when a pixel's Row switch is closed, enabling its source follower to provide a signal through line 114, the source follower's operation may be affected by the modified bias current. If the photocurrents increase the bias current, the source follower may saturate at a lower threshold, and if the photocurrents are sufficiently large, the source follower gain can be substantially reduced. Illustratively, if a pixel normally provides a signal from 0 to 1 volt, its signal might reach a maximum at 0.6 or 0.7 volts instead of 1 volt. Also, gain may be reduced for all pixels in a column. Therefore, the column's readout circuitry may receive reduced signals for all pixels in the column, forming a vertical line in an image read out from the array.
Vertical lines and other effects resulting from intense illumination of pixels in a row/column array or other configuration are referred to herein as “bright light effects”. As used herein, “light” includes all wavelengths of electromagnetic radiation, and the term “bright light” is used herein to encompass any source of electromagnetic radiation at sufficient intensity to produce a bright light effect, regardless of the radiation's wavelength distribution. An extended bright light effect such as a vertical line, if produced by an image of the sun, may be called “sun smear”.
In addition to row/column arrays, the spurious current problem can also arise in other arrays and configurations in which light sensing pixels or other signal-providing elements provide signals through readout lines.
The invention provides techniques that alleviate the spurious current problem.